No. Can't you log in as the admin user (who was created at installation time)? If yes, log in and add the new user to the sudo group, where's the problem?
If no, do you have physical access to the machine? If yes, boot from a Ubuntu CD, mount your harddrive and add the user to the group manually. For example, assuming that
/dev/sdb1 is the partition on your hard drive that is mounted as root (
/), then after booting the live CD, do:
mount /dev/sdb1 /mnt
(yeah, you can do that all from a GUI as well)
A simple text editor opens. Search for the line starting with "sudo". It may look something like this:
oldadmin is the user name of the admin. If the new user's login name is
newuser, modify that line to
save the file and reboot the computer. The
newuser should have now the privilege to run sudo.
If you do not have physical access to the machine, you lost your administrative password and the root is not permitted to log in, then you have a problem, sir.